The invention relates to a color image forming apparatus using the electrophotography technique. More specifically, the invention relates to a so-called tandem-type color image forming apparatus, wherein images are formed on respective image carriers with light scanned thereon in a primary scanning direction by a rotary polygon mirror while rotating them in a secondary scanning direction, and the formed images are superimposed one after another on an object of transfer to be moved in the secondary scanning direction. In particular, the invention relates to a countermeasure against a misregistration in colors of the images.
As described in, e.g., Japanese Patent Publication Nos. 5-270051A, 6-286226A, and 2000-347116A, a tandem-type color image forming apparatus of single rotary polygon mirror type has been known. A plurality of laser beams are deflected to perform a primary scanning by use of a single rotary polygon mirror, whereby the beams are separated from each other by a beam separation optical system constituted of a reflection mirror or the like. The thus-separated beams are guided to image carriers of corresponding colors, thereby forming images (latent images). As described in, e.g., Japanese Patent Publication Nos. 62-242471A and 7-97244B, there has also been known a tandem-type color image forming apparatus of multiple rotary polygon mirror type, wherein a laser scanning unit, comprising a laser light source, a rotary polygon mirror, and a scanning optical system, is provided for each image carrier. A rotary polygon mirror subjects a laser beam to deflection scanning (primary scanning) on a per-color basis, thereby forming an image on an image carrier of corresponding color.
In a color image forming apparatus of either of the above types, the images (latent images) formed on the respective image carriers are developed by a developer. The images are superimposed one after another on objects of transfer (intermediate transfer members or recording medium, such as paper) to be moved in a direction in which the image carriers are to be rotated (i.e., a secondary scanning direction). If the images formed on the respective image carriers are not correctly registered with each other, a misregistration arises in colors, thus deteriorating picture quality.
To solve the problem, as shown in FIG. 15, there has already been proposed a color image forming apparatus (as described in Japanese Patent No. 2608080), wherein a plurality of rotational image carriers 1(Y, M, C) are subjected to scanning in a primary scanning direction (an axial direction of the image carriers, that is, in the direction of a generating line), thereby forming images on the respective image carriers 1(Y, M, C). The images are sequentially superimposed on paper S to be moved in a secondary scanning direction in which the image carriers are to rotate, thereby forming a color image.
A scanning period of each scanning surface of a rotary polygon mirror 2 is taken as Ts (see FIG. 16). A time T1 is defined as the time lapsing while an image formed on the upstream image carrier 1(Y) from among the plurality of image carriers, for example, 1(Y) and 1(M) is moved from an image formation position 1a(Y) to a transfer position 1b(M) located between the downstream image carrier 1(M) and an object of transfer.
A time T2 is defined as the time lapsing while the image formed on the downstream image carrier 1(M) from among the plurality of image carriers is moved from an image forming position 1a(M) to a transfer position 1b(M) at which the image is to be transferred onto an object of transfer.
Under these conditions, the plurality of image carriers 1(Y, M, C) are arranged such that T1xe2x88x92T2≅nTs stands. The publication states that the range of xe2x80x9c≅xe2x80x9d is about xc2x120% of Ts. Here, xe2x80x9cnxe2x80x9d represents natural numbers.
In FIG. 15, L(Y), L(M), L(C) denote laser beams to be used for subjecting the image carriers 1(Y), 1(M), 1(C) to primary scanning. A belt 4 is an object of transfer to be used for transporting paper S in a secondary scanning direction.
The related-art apparatus yields the following advantage.
Namely, as shown in FIG. 16A, each of rotary polygon mirrors 2(Y), 2(M), 2(C) is assumed to be an octagonal prism which is rotated in the direction of the arrow and has scanning surfaces P1 through P8. The image carrier 1(Y) is sequentially subjected to primary scanning through use of the scanning surfaces P1 through P8 of the rotary polygon mirror 2(Y) (see FIG. 15), thereby forming images p1 to p8 on the image carrier 1(Y). The images p1 through p8 are transferred to the paper S. As shown in FIG. 16C, the image carrier 1(M) is also sequentially subjected to primary scanning through use of the scanning surfaces P1 through P8 of the rotary polygon mirror 2(M) (see FIG. 17), thereby forming images p1 through p8 on the image carrier 1(M). The images p1 through p8 on the paper S and the images p1 through p8 formed on the image carrier 1(M) are substantially in phase with each other at a transfer position 1b(M) at which images are to be transferred from the downstream image carrier 1(M) to an object of transfer. Hence, misregistration of colors is prevented. In this case, there is prevented occurrence of a great misregistration between the images p1 through p8 formed on the image carrier 1(Y) and the images p1 through p8 formed on the image carrier 1(M) with respect to the secondary scanning direction (i.e., the direction in which the object of transfer S is to be moved).
FIGS. 17A and 17B are views schematically showing writing of an image onto the image carrier 1 performed by the rotary polygon mirror 2.
As shown in FIG. 17A, the rotary polygon mirror 2, a shaft 2a of the mirror 2, and a support member 2b of the mirror 2 involve production errors. A rotation center O2 of the rotary polygon mirror 2 is inclined with respect to an ideal rotation center O1 (an angle of inclination is represented by xcex8).
For this reason, an image to be written (formed) on the image carrier 1 by the laser beam reflected from respective scanning surfaces of the rotary polygon mirror 2 will become deviated from an ideal writing position p0 (see FIG. 17B) with respect to the direction of movement of the image carrier 1 (i.e., the direction designated by an arrow; that is, the secondary scanning direction). Here, reference numeral 3 designates a scanning lens.
A maximum misregistration arises in the secondary scanning direction between images formed by mutually-opposing scanning surfaces (e.g., even-numbered surfaces which are 180xc2x0 out of phase with each other). For instance, the rotary polygon mirror 2 is assumed to be an octagonal prism (see FIG. 16A) having scanning surfaces P1 through P8. As illustrated, maximum misregistration arises between an image p1 formed by a scanning light L (P1) reflected by the scanning surface P1 and an image p5 formed by a scanning light L (P5) reflected by the scanning surface P5. Similarly, maximum misregistration arises between images p2 and p6; between images p3 and p7; and between images p4 and p8.
Such a misregistration in the secondary scanning direction is also caused by inclination of the scanning surfaces with respect to the rotation center O2 of the rotary polygon mirror 2.
According to the related-art technique shown in FIG. 15, the plurality of image carriers 1(Y), 1(M), 1(C) are arranged such that T1xe2x88x92T2≅nTs stands. Hence, misregistration between colors becomes unlikely to arise. Specifically, as shown in FIGS. 16C and 16D, great misregistration becomes unlikely to arise in the secondary scanning direction between the images p1 through p8 formed on the image carrier 1(Y) and the images p1 through p8 formed on the image carrier 1(M). According to the related-art technique, the related-art apparatus is constructed so as to achieve the relationship T1xe2x88x92T2≅Ts. Hence, there may arise a problem of images formed on different scanning surfaces of the rotary polygon mirror 2 being superimposed on each other. Therefore, as shown in, e.g., FIGS. 16C and 16D, the image p1 formed by the scanning surface P1 of the rotary polygon mirror 2 may be superimposed on the image p5 formed by the scanning surface P5 of the same.
When such a problem has arisen, misregistration attributable to an inclination of the rotation center O2 of the rotary polygon mirror 2 may arise between images.
As is evident from the above descriptions, the related-art technique shown in FIG. 15 has a problem of difficulty in reducing misregistration between images that would be attributable to an inclination of the rotation center O2 of the rotary polygon mirror 2.
Such a problem arises in a similar manner even when a rotary polygon mirror is constituted of a single rotary polygon mirror.
It is therefore an object of a color image forming apparatus capable of lessening misregistration between images.
In order to achieve the above object, according to the invention, there is provided a color image forming apparatus, comprising
a transporting path, through which a recording medium is transported;
a plurality of rotary image carriers, arranged in the transporting path; wherein
at least one rotary polygon mirror, which rotates to reflect light so as to scan an outer peripheral surface of each of the image carriers in a first direction when each of the image carriers is rotated in a second direction perpendicular to the first direction, so that a monochrome latent image is formed on the outer peripheral surface of each of the image carriers to be developed and superimposedly transferred onto a transferred object to form a color image thereon,
wherein the image carriers are arranged so as to satisfy the following expression:
xe2x88x92Ts/2 less than t1+t2xe2x88x92t3xe2x88x92nTp less than Ts/2 
where Ts is a time period of scanning performed by each of reflecting surfaces of the polygon mirror;
Tp is a time period of one rotation of the polygon mirror;
t1 is a time period required for a first latent image formed on a first image carrier among the image carriers is developed and transferred to the transferred object at a first transferring position;
t2 is a time period required for the first image is traveled from the first transferring position to a second transferring position at a downstream of the transporting path;
t3 is a time period required for a second latent image formed on a second image carrier adjacent to the first image carrier is developed and transferred to the transferred object at the second transferring position; and
n is a natural number.
In such a configuration, the image formed by a certain scanning surface of the rotary polygon mirror and the image formed on the downstream image carrier by the same scanning surface of the rotary polygon mirror are inevitably overlapped on one another. A misregistration developing between images due to warpage of the scanning surfaces is cancelled. Therefore, a misregistration between images is diminished further
According to the invention, there is also provided a color image forming apparatus, comprising:
a transporting path, through which a recording medium is transported;
a plurality of rotary image carriers, arranged in the transporting path; wherein
at least one rotary polygon mirror, which rotates to reflect light so as to scan an outer peripheral surface of each of the image carriers in a first direction when each of the image carriers is rotated in a second direction perpendicular to the first direction, so that a monochrome latent image is formed on the outer peripheral surface of each of the image carriers to be developed and superimposedly transferred onto a transferred object to form a color image thereon,
wherein the image carriers are arranged so as to satisfy the following expression:
|t1+t2xe2x88x92t3xe2x88x92nTp| less than [(Np/2)xe2x88x920.5]Tp/Np 
where Np is the number of reflecting surfaces of the polygon mirror which is an even number not less than 4;
Ts is a time period of scanning performed by each of the reflecting surfaces of the polygon mirror;
Tp is a time period of one rotation of the polygon mirror;
t1 is a time period required for a first latent image formed on a first image carrier among the image carriers is developed and transferred to the transferred object at a first transferring position;
t2 is a time period required for the first image is traveled from the first transferring position to a second transferring position at a downstream of the transporting path;
t3 is a time period required for a second latent image formed on a second image carrier adjacent to the first image carrier is developed and transferred to the transferred object at the second transferring position; and
n is a natural number.
Alternatively, there is also provided a color image forming apparatus, comprising;
a transporting path, through which a recording medium is transported;
a plurality of rotary image carriers, arranged in the transporting path; wherein
at least one rotary polygon mirror, which rotates to reflect light-so as to scan an outer peripheral surface of each of the image carriers in a first direction when each of the image carriers is rotated in a second direction perpendicular to the first direction, so that a monochrome latent image is formed on the outer peripheral surface of each of the image carriers to be developed and superimposedly transferred onto a transferred object to form a color image thereon,
wherein the image carriers are arranged so as to satisfy the following expression:
|t1+t2xe2x88x92t3xe2x88x92nTp| less than [(Np/2)xe2x88x920.5]Tp/Np 
where Np is the number of reflecting surfaces of the polygon mirror which is an odd number not less than 3;
Ts is a time period of reflecting performed by each of the scanning surfaces of the polygon mirror;
Tp is a time period of one rotation of the polygon mirror;
t1 is a time period required for a first latent image formed on a first image carrier among the image carriers is developed and transferred to the transferred object at a first transferring position;
t2 is a time period required for the first image is traveled from the first transferring position to a second transferring position at a downstream of the transporting path;
t3 is a time period required for a second latent image formed on a second image carrier adjacent to the first image carrier is developed and transferred to the transferred object at the second transferring position; and
n is a natural number.
In such configurations, occurrence of the following situation is at least prevented. Specifically, an image formed on an upstream image carrier by a certain scanning surface of the rotary polygon mirror and an image formed on a downstream image carrier by an opposing scanning surface of the rotary polygon mirror are superimposed on each other.
Therefore, there is prevented occurrence of maximum misregistration between images, which would otherwise be attributed to an inclination of the rotation center of the rotary polygon mirror. Consequently, a misregistration arising between images is diminished.
In the above configurations, a plurality of polygon mirrors may be provided so as to be associated with the image carriers in a one-by-one manner.
In the above configurations, the transferred object may be an intermediate transfer belt which temporarily retains the color image to be transferred to the recording medium.
Alternatively, the transferred object may be the recording medium.